What is the cheapest JUNK FOOD?

I know this seems counter productive but I'm really looking for the absolute cheapest most tastiest and satisfying junk food out there. I'm looking for BULKY, FILLING, tasty and probably high calorie junk food.

EVERY LIST I can find about cheap food is only for health food! I WOULD buy healthy foods (and I often do) but they just plainly don't get eaten and then go to waste. Costing me way more money.

It seems silly but my three person family is burning through a 300-400 food budget and it seems the majority of it is on fancy name brand junk foods.

Before anybody says a word about obesity.... my son is exceptionally small. He's in the 5th percentile for height and weight, (his dad and I are both also at about 19.5 BMI), so I'm actually more worried about my kid losing weight than gaining it.